1. Field of the Invention
This invention relates generally to safety pressure relief devices, and more particularly, but not by way of limitation, to a safety pressure relief device designed for positive rupture in both the normal and reverse rupture modes at pre-determined normal and reverse rupture pressure differentials, with the pre-determined reverse rupture pressure differential being relatively lower than that which could previously be predictably achieved by the prior art.
2. Description of the Prior Art
A variety of safety pressure relief devices of the rupture disk type have been developed. Generally, these devices include a rupture disk supported between a pair of complementary supporting members or flanges which are in turn connected to a relief connection in a vessel or system containing fluid pressure. When the fluid pressure within the vessel or system exceeds the design pressure of the rupture disk, rupture occurs allowing fluid pressure to be relieved from the vessel or system.
Although prior art devices have been designed for failure in either direction, see e.g. U.S. Pat. No. 3,091,359 to Wood, the rupture disk devices of the prior art have typically been designed for failure primarily in one direction. That is if the rupture disk is to be installed in a pressurized system between first and second pressure zones, and it is contemplated that the excess pressure will normally occur in the first zone and be desirably vented to the second zone, the rupture disk is designed to be installed at such an orientation that it will fail due to excess pressure on the side of the disk adjacent the first zone. This is referred to as the normal mode of failure.
A problem sometimes occurs, however. Although the system may be designed so that in its normal operation the excess pressure will occur in the first zone, a situation may occur where the excess pressure is in the second zone. For example, a typical installation of the relief device of the present invention is to install it for normal venting of excess pressure from inside a petrochemical storage tank to the atmosphere. There normally is a pressure inside the tank greater than atmospheric pressure, due to the fluid stored therein, and the disk may normally be installed for rupture due to excess pressure within the tank. However, the situation may occur where the fluid is being drawn from the tank thereby pulling a vacuum in the tank and causing the excess pressure to be external of the tank. In such a situation it is desirable that this excess pressure external of the tank be relieved so as to prevent the tank from collapsing due to the internal vacuum. Although relatively large diameter rupture disks have sometimes been used by the prior art to protect these tanks from an internal vacuum, many of the more conventionally sized rupture disk designs of the prior art cannot be satisfactorily controlled when failing in this reverse direction and thereby may permit unsafe conditions to occur.
One previous attempt to provide a relief apparatus designed to relieve excess pressure in both directions is that shown in U.S. Pat. No. 3,091,359 to Wood, mentioned above. That device has certain limitations which are overcome by the present invention. The minimum rupture pressures which can predictably be achieved by the `359` device are considerably higher than those of the present invention, as will be explained in more detail later. Also, experience has shown that the `359` apparatus will provide a total rupture of both its disks in the reverse mode only if the design rupture pressure in the normal direction is not greater than twice the design rupture pressure in the reverse mode. Otherwise, the normal mode rupture disk will generally not reverse and rupture when the reverse mode rupture disk ruptures.
These problems are overcome by the rupture disk assembly of the present invention by providing a composite disk designed for positive rupture in both the normal and reverse directions at different pre-determined normal and reverse pressure differentials. A novel structure is provided which assures predictable rupture in the reverse mode at pressure differentials considerably lower than can be achieved by the prior art devices. Also, by combining two of the rupture disks of the present invention face to face, a combination disk is provided which assures rupture in either direction. For proper operation of this combination the two rupture disks should be designed to rupture at the same pressure differentials.
In one embodiment of the present invention, those portions of the assembly providing for rupture in the normal direction are provided by a conventional concave-convex rupture disk having a plurality of radial slits therein in combination with a flexible sealing member. Similar apparatus for failure in the normal mode are described in U.S. Pat. No. 2,953,279 to Coffman and U.S. Pat. No. 3,698,598 to Wood et al. This embodiment of the present invention, when failing in the normal direction, operates in a similar manner to Coffman. When the concave-convex disk ruptures outwardly, splitting into a plurality of triangular petals, the flexible sealing member is burst by the high normal pressure differential.
A particular problem that must be dealt with when designing a composite rupture disk for failure in both the normal and reverse directions is that failure in the reverse direction is typically caused by the drawing of a vacuum in one pressure zone and the pressure differentials encountered are relatively low, although they are sufficient to damage large structures such as oil storage tanks. The problem with these low pressure differentials is the necessity for designing the rupture disk to fail positively and predictably at a given low reverse pressure differential.
The reverse mode rupture disk, of the present invention, having the flexible sealing member on the side adjacent the second pressure zone and the circular cutting blade on the side adjacent the first pressure zone, provides this required positive and predictable rupture at the desired low reverse pressure differential. Neither the Coffman or Wood et al devices provide a means for positive rupture of the flexible sealing member at these low pressure differentials. U.S. Pat. No. 2,095,828 to Nerad shows a circular cutting blade, but it is not used in conjunction with a flexible sealing member and does not involve relief of fluids at low pressure differentials.